A thermal battery is a thermally-activated primary reserve battery which has a solid state inorganic salt electrolyte having no ion conductivity between a negative electrode and a positive electrode at room temperature, and is activated by liquefying the electrolyte using heat of a heat generating material. Well-known thermal batteries are stored in a state in which the batteries are not activated at room temperature, and are activated upon receiving heat at a high temperature of 300° C. to 400° C. to perform a function of supplying power.
A thermal battery is advantageous in that long-term storage characteristics are excellent because the battery does not self-discharge even when stored for a long period of 10 years or more, reliability characteristics, such as being hardly affected by a vibration and impact, of the battery are excellent, and maintenance activities are unnecessary. Thermal batteries have been mainly used for military applications, such as power supplies of detonators for guided missiles, rockets, or the like, in which properties of a thermal battery can be effectively used.
Generally, a thermal battery has a structure in which a plurality of unit cells composed of a positive electrode, a negative electrode, a solid electrolyte, and a heating material are stacked, packaged with an insulator for blocking heat from the outside, and inserted into a case to be sealed, and is provided with an igniter configured to ignite the heating material of the unit cell and a connector for connection with an external device. Here, the positive electrode, the negative electrode, the solid electrolyte, and the heating material constituting the unit cell may each have a disk-shaped pellet form, and such elements in the pellet form may be sequentially stacked to form the unit cell. When the thermal battery needs to be operated, the heating material is activated or burned by the igniter to generate heat, and the heat causes liquefaction of the solid electrolyte to generate an electromotive force, thereby operating as a battery.
However, an operating temperature of a thermal battery is a very high temperature, and when the operating temperature is lowered, the molten solid electrolyte changes back to a solid and cannot perform a battery function, which is disadvantageous.
In well-known thermal batteries, a solid electrolyte uses a lithium eutectic salt such as LiCl—KCl, LiBr—KBr—LiF, LiCl—LiBr—LiF, LiCl—LiBr—KBr, or the like. However, since such a lithium eutectic salt has a melting point of 300° C. or higher, a conventional thermal battery requires a high operating temperature of 300° C. or higher (melting point or higher) for operation and a high-efficiency and high-cost insulating structure capable of maintaining a state of a melting point or higher so that a battery function can be maintained. This becomes a factor increasing the manufacturing cost of a thermal battery.